matt_the_blat Posted January 28, 2009 Report Share Posted January 28, 2009 I've just been looking at some of the tech-specs for my bike (02 Aprilia Mille) - the (official Aprilia) manual says (and I've checked for typos): inlet opens 25deg before TDC inlet closes 58deg after BDC exhaust opens 64deg before TDC exhaust closes 15deg after BDC Now, not forgetting that a four stroke cycle is 720deg (not 360), unless I'm being a spaz (entirely possible) that means the inlet and exhaust valves overlap, i.e. they're both open at the same time So why is that then? Is the idea that the incoming fresh air helps to push out the old exhaust gases? Also, why does the inlet valve open before TDC? Surely that would mean it's open during the exhaust stroke Why's that then? Wouldn't it end up pushing exhaust gas out through the inlet port? (The reason I'm asking all this is I want to know how much air (ideally the mass, but volume would do) goes into the cylinder during each cycle - unfortunately I don't think it's as straightforward as volume of air = cylinder capacity but I think I can work something out from the above) Any suggestions/explanation greatly appreciated - cheers, Matt Link to comment Share on other sites More sharing options...
ronenige Posted January 28, 2009 Report Share Posted January 28, 2009 With the inlet opening "early" this takes advantage of the fact that the air in the inlet tract may have some momentum remaining after the last induction. By opening the inlet, this utilises this inertia to "stuff" more charge into the cylinder (a mild forced induction effect). near enough , the time when both valves are open together is refered to as overlap ,in this case 40 degrees ,this has quite a marked effect on the power delivery of the motor , one reason moving cams 3 or 4 degrees can have such an effect is it may alter the overlap by 10 % or more , it's obviously a long and complex subject and the effects vary from one engine design to another , but what zak says is near enough Link to comment Share on other sites More sharing options...
ronenige Posted January 29, 2009 Report Share Posted January 29, 2009 "Zakalwe in sensible post shocker...." IIRC, inlet tract lengths are tuned to maximise this effect. When the inlet valve snaps shut it sends a shock wave back up the tract. This shock wave can be reflected back down the inlet tract (from inside the airbox or the edge of the inlet trumpet) to arrive just as the vale re-opens (at the point in the rev range that makes maximum power). This returning shockwave has the effect of ramming more air/fuel into the cylinder.... Clever people, these engine designers right again young zak , usually a shorter inlet tract for high revs and a longer one for lower revs hence the reason for yamaha's YCCI and similar devices Link to comment Share on other sites More sharing options...
matt_the_blat Posted January 29, 2009 Author Report Share Posted January 29, 2009 "Zakalwe in sensible post shocker...." IIRC, inlet tract lengths are tuned to maximise this effect. When the inlet valve snaps shut it sends a shock wave back up the tract. This shock wave can be reflected back down the inlet tract (from inside the airbox or the edge of the inlet trumpet) to arrive just as the vale re-opens (at the point in the rev range that makes maximum power). This returning shockwave has the effect of ramming more air/fuel into the cylinder.... Clever people, these engine designers As an aerodynamicist <geek> I don't like the term shockwave there (that's something very different) - pressure wave will do though </geek> Thanks for the info guys Link to comment Share on other sites More sharing options...
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